Telecommunication networks of today generally employ optical fibers for signal transmission. Optical signals are transported long distances on one or a plurality of optical carriers and features like long legs and power splitting necessitate amplification or regeneration of weakened signals. Optical amplification is often the most desirable option, since it offers direct amplification without problematic conversion between optical and electric signals.
Optical amplifiers typically comprise a comparatively short amplifier fiber doped with a rare-earth metal or another substance that is capable of fluorescing. Light of the same wavelength as the input signals is pumped into the amplifier fiber by a pump laser and absorbed photons cause electrons of the rare-earth atoms to jump to a temporary excited stage. As the electrons decay, photons are released and added to the input signal, increasing its gain.
For the conventional (C) band, it is well known to use erbium doped fiber amplifiers (EDFA), which has been thoroughly researched. The increasing demand for bandwidth in wavelength division multiplexing (WDM) optical communication systems has lead towards extending the transmission bands outside the C-band. Below the C-band, there is the so-called S-band (1460-1520 nm) for which fiber amplifiers doped with thulium ions (Tm3+), presenting a 1470 nm emission band, are suitable. Thulium doped fiber amplifiers (TDFA) provide excellent positioning in the supporting band as well as a high power conversion efficiency originating from the rare earth based nature.
TDFA thus hold the potential to enable larger transmission capacity in the future. Several glass systems have been researched, highlighting silicate, fluoride and, most recently, tellurite glasses doped with thulium ions (Tm3+). However, there are some problems that have to be overcome before an efficient optical amplifier doped with Tm3+ can be fabricated. Thulium doped glass systems involve four energy levels, including the 3H4 and 3F4 levels, respectively. These glasses are limited by the fact that lifetime of the lower level is larger than that of the upper level, which means that basic Tm-fibers are inoperative as amplifiers. Therefore, in order to fabricate a well-functioning optical amplifier using thulium as dopant, the lower level needs to be depopulated. Three methods have been used for both silicate and fluoride glasses: up conversion pumping; laser oscillation of band at 1.8 μm; and co-doping with holmium ions (Ho3+). Transition in 1470 nm is not available in silica glasses because 3H4 level drops by predominantly non-radioactive mechanisms. While all three processes were successfully demonstrated in fluoride glasses, such glasses present a poor chemical durability and are very difficult to fabricate as low loss optical fibers.
Consequently, a glass system that combines the optical properties of fluoride glasses with the physical properties of silica glasses would be very desirable, and these requirements are met by the tellurite (Te) glasses. Tellurite glasses have been found to provide a broader thulium emission spectrum than other glasses. Hence, tellurite glass optical fibers result in broader band optical amplifiers for WDM, enabling an increased number of wavelength optical channels. At the same time, the solubility of the rare earth element is comparatively high, resulting in a very high gain per unit length. Due to the high doping level, amplifiers comprising tellurite glass only require centimeter long optical fibers.
A key issue in order to provide efficient tellurite based TDFAs is thus to overcome the fast lifetime in the signal transition band (3H4-3F4) to achieve a good inversion for the amplification process. Several tellurite glass compositions doped with thulium have been proposed in the prior art. In [1], for example, tellurite glasses of the TeO2—ZnO—Na2O family doped with Tm3+ ions are described. The glasses have significant advantages, mainly when combined with tellurite glasses doped with Er3+ ions. However, the authors did not manage to depopulate 3F4 lower level, which lifetime is always larger than that of the 3H4 upper level. Document [2] also concern glasses of the TeO2—ZnO—Na2O family. Here, the authors failed to depopulate the lower level, even with co-doping of Ho3+ ions or Tb3+ ions. The lifetime of the 3H4 upper level is always shorter than that of 3F4 lower level with these glasses.
Accordingly, problems associated with rare earth doped tellurite glasses for optical fiber amplifiers remain. To our knowledge, no tellurite glass with satisfactory transition lifetimes has been disclosed in the prior art and there is a need for an improved glass composition.